Existing bone anchored hearing aids include a transducer or vibrator that has a resonance frequency F. This frequency is defined as the resonance frequency of the device when it is measured in a standard skull simulator, type TU-1000 (ref: Håkansson B, Carlsson P. Scand Audiol. 1989; 18(2):91-8) To dampen the resonance frequency, the existing bone anchored hearing aids uses an electronic notch filter with a notch frequency F1 that corresponds to the resonance frequency F of the hearing aid transducer. In this way the resonance is dampened and the frequency response becomes more flat.
The existing notch filter damping is practical when measuring the resonance frequency of the bone anchored hearing aid on the skull simulator. The drawback with the existing notch filter damping is that it dampens the resonance at the resonance frequency of the device when it is connected to the skull simulator. The resonance frequency of the vibrator is however not the same on a patients head as on the standard skull simulator, due to the difference in mechanical impedance between a skull simulator and a human head. And in fact there are differences of the mechanical impedance between different patients, so there is a difference in resonance frequency of the transducer when it is connected to different patients.
Since the current bone anchored hearing aids has a notch filter frequency adapted to the resonance frequency on the skull simulator, there will be a less optimal frequency response for the patient when the device is connected to the patient instead.
An example of a prior art bone anchored hearing aid is presented in WO 2005/029915 A1. Here differences between the resonance frequencies from one hearing aid to the other is taken into account, but the differences due to the different properties of patients heads or skull bone structure is not accounted for, and as a result the hearing aid will be better suited for some patients than for others.